Negative-pressure polymorphs made by heterostructural alloying

Abstract

The ability of a material to adopt multiple structures, known as polymorphism, is a fascinating natural phenomenon. Various polymorphs with unusual properties are routinely synthesized by compression under positive pressure. However, changing a material's structure by applying tension under negative pressure is much more difficult. We show how negative-pressure polymorphs can be synthesized by mixing materials with different crystal structures-a general approach that should be applicable to many materials. Theoretical calculations suggest that it costs less energy to mix low-density structures than high-density structures, due to less competition for space between the atoms. Proof-of-concept experiments confirm that mixing two different high-density forms of MnSe and MnTe stabilizes a Mn(Se,Te) alloy with a low-density wurtzite structure. This Mn(Se,Te) negative-pressure polymorph has 2× to 4× lower electron effective mass compared to MnSe and MnTe parent compounds and has a piezoelectric response that none of the parent compounds have. This example shows how heterostructural alloying can lead to negative-pressure polymorphs with useful properties-materials that are otherwise nearly impossible to make.

Fig. 1. Stabilization of negative-pressure polymorphs in heterostructural alloys.

(A) Schematic illustration of α-, β-, and γ-polymorph enthalpies ΔH as a function of their respective atomic volume V and polymorph energies ΔE_A for a model compound A. Stabilization of lower-density polymorph γ would require negative pressure (p = −∂H/∂V < 0). (B) Polymorph enthalpies as a function of the alloying composition x in an alloy system A_1−xB_x at equilibrium volume. Because of a smaller nonideal component of the mixing enthalpy ΔH_Ω,γ < ΔH_Ω,α, the lower-density γ polymorph can become energetically favorable for intermediate alloying concentrations, if entropic stabilization or kinetic barriers prevent phase separation into α and β. (C) Grayscale projection of the minimum enthalpy ΔH_min(V,x) among the considered polymorphs outlines the basins of attraction for the three considered structures. The smaller bowing of the γ-polymorph enthalpy enables the stabilization of this high-volume structure by controlling the alloying concentration. a.u., arbitrary units.

Fig. 2. Stabilization of the WZ polymorph of the Mn(Se,Te) alloy.

(A) Calculated minimum enthalpy ΔH_min of MnSe_1−xTe_x alloys on the color scale as a function of volume V and composition x. For intermediate compositions, the higher-volume WZ structure becomes energetically favorable compared to lower-volume RS and NC structures, like under negative-pressure conditions. (B) Measured false-color plot of XRD intensities as a function of composition for the MnSe_1−xTe_x films deposited at 320°C. For intermediate compositions, the Mn(Se,Te) films crystallize mostly in the WZ structure, whereas the MnSe and MnTe parent compounds crystallize in RS and NC structures.

Fig. 3. Experimental crystal structure analysis of MnSe_1−xTe_x alloys.

(A) Synchrotron XRD measurements of MnSe_0.5Te_0.5 thin films grown on glass at 320°C substrate temperature [black circle with white boarder in (B)] confirm the stabilization of the high-enthalpy, low-density WZ polymorph. Trace amounts of MnTe NC (*) and MnSe RS (#) could be present in the film. The top and the bottom panels show the simulated XRD patterns of MnSe and MnTe in WZ and other structures, and the dashed lines are extrapolations of the WZ peaks. (B) Color-scale map of the WZ phase fraction for the sputter-deposited MnSe_1−xTe_x thin films on glass. For intermediate compositions and lower deposition temperatures, the MnSe_1−xTe_x films crystallize predominantly in the WZ structure with some RS- and NC-type impurities. Shaded areas represent single-phase regions of RS-MnSe and NC-MnTe determined by the disappearing phase method.

Fig. 4. Theoretical electronic and crystal structures of the Mn(Se,Te) materials.

(A) MnSe in the RS crystal structure, (B) MnSe_0.5Te_0.5 in the WZ crystal structure, and (C) MnTe in the NC crystal structure. The larger band gap and smaller electron effective mass of the WZ-Mn(Se,Te) alloy result from weaker p-d hybridization of the Mn d-states of e_g symmetry with the Se,Te p-states. In turn, the weaker hybridization is caused by tetrahedral coordination of the lower–atomic density WZ structure compared to the octahedral coordination of the RS-MnSe and NC-MnTe structures.